GBA: More I/O register cleanup
GBA: More I/O register cleanup

Anty-Lemon/mgba,Iniquitatis/mgba,Touched/mgba,fr500/mgba,Anty-Lemon/mgba,Touched/mgba,jeremyherbert/mgba,jeremyherbert/mgba,sergiobenrocha2/mgba,Anty-Lemon/mgba,AdmiralCurtiss/mgba,fr500/mgba,libretro/mgba,jeremyherbert/mgba,libretro/mgba,Anty-Lemon/mgba,mgba-emu/mgba,Iniquitatis/mgba,sergiobenrocha2/mgba,sergiobenrocha2/mgba,mgba-emu/mgba,mgba-emu/mgba,fr500/mgba,sergiobenrocha2/mgba,MerryMage/mgba,sergiobenrocha2/mgba,fr500/mgba,MerryMage/mgba,jeremyherbert/mgba,MerryMage/mgba,libretro/mgba,iracigt/mgba,iracigt/mgba,AdmiralCurtiss/mgba,libretro/mgba,Iniquitatis/mgba,Iniquitatis/mgba,AdmiralCurtiss/mgba,iracigt/mgba,iracigt/mgba,mgba-emu/mgba,Touched/mgba,libretro/mgba